[Proj] Meaning of 7-parameter transformation rotations

Clifford J Mugnier cjmce at lsu.edu
Tue Dec 13 01:37:34 EST 2005


It's since been published, I think, but when I was in Trinidad on my
"project,"  WGS84 didn't even exist.  The WGS84 7-parameter shift values
weren't published when I wrote my column, either.

Spring chicken, I ain't.  :-)  Been around the block and the geoid a time
or two, though ... With regard to EPSG stuff, only Roger Lott can answer
that question.

Merry Christmas!



On 11/30/05, Clifford J Mugnier <cjmce at lsu.edu> wrote:
> Gentlemen:
> Note that the SENSE of the rotations have two common usages.  The
> Standard versus the European Standard differ!  To transform in the same
> direction, the signs are OPPOSITE from one "standard" to another.
> Generally speaking, a 7-parameter  transformation, whether it is a
> Bursa-Wolfe or a Molodensky, MUST have a "test point" provided so that
> user knows which sense the rotations need for the desired transformation
> direction.
> The Americans, Australians, and Luxemborgs use one SENSE, the Europeans
> the other.  In regard to the rest of the world, it depends on which
> brand-name of GPS Geodetic Processing software was used to derive the
> parameters from some collection of data points.  See my monthly column
> such important minutiae for country to country considerations.


Pursuant to this point, would it be fair to assume that if
you don't say otherwise, that you are using the "American"
sense?  For instance, I find myself needing a transformation
for Trinidad 1903 datum, and would like to use the one you
published in your column.

Also, it is a matter of some challenge to me that you don't
provide WGS84 test values for your test point (ie. the
point of origin on Naparima Hill) for me to test, though it is
handy that you provide that location in both the Naparima 1972
datum and Trinidad 1903 datum.


BTW, EPSG publishes +towgs84=-61.702,284.488,472.052
for Trinidad 1903 to WGS84 which seem to different significantly
from your solution of -33.25, +232.675, +484.542.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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